That's exactly a situation where signage is mandatory (and, I think, legally required) to say what the current rule is. The hard-coded traffic law that would apply here is "obey the traffic signs".

The point I was making is that these cars should absolutely not be "learning" what rules to follow by observing what other drivers actually do. When there is no clear signage, there are well-defined laws about what drivers need to do and self-driving cars need to follow those even if human drivers don't.